Ataxia From Greek α- (used as a negative prefix) + -τάξις (order), meaning "lack of order". It is a movement disorder, a neurologicalsign and symptom consisting of gross incoordination of muscle movements. Ataxia is an aspecific clinical manifestation implying dysfunction of parts of the nervous system that coordinate movement, such as the cerebellum. Several possible causes exist for these patterns of neurological dysfunction. The "International Ataxia Awareness Day" is celebrated on September 25th each year.

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The term cerebellar ataxia is employed to indicate ataxia due to dysfunction of the cerebellum. This causes a variety of elementary neurological deficits, such as antagonist hypotonia, asynergy, dysmetria, dyschronometria, and dysdiadochokinesia. How and where these abnormalities manifest depend on which cerebellar structures are lesioned, and whether the lesion is bilateral or unilateral.
Vestibulo-cerebellar dysfunction presents with postural instability, in which the person tends to separate the feet on standing to gain a wider base, and avoid oscillations (especially posterior-anterior ones); instability is therefore worsened when standing with the feet together (irrespective of whether the eyes are open or closed: this is a negative Romberg's test).
Spino-cerebellar dysfunction presents with a wide-based "drunken sailor" gait, characterised by uncertain start and stop, lateral deviations, and unequal steps.
Cerebro-cerebellar dysfunction presents with disturbances in carrying out voluntary movements, including intention tremor (coarse trembling, accentuated over the execution of voluntary movements, possibly involving the head and eyes as well as the limbs and torso), peculiar writing abnormalities (large, unequal letters, irregular underlining), and a peculiar pattern of dysarthria (slurred speech, sometimes characterised by explosive variations in voice intensity despite a regular rhythm).

The term sensory ataxia is employed to indicate ataxia due to loss of proprioception (sensitivity to joint and body part position), which generally depends on dysfunction of the dorsal columns of the spinal cord, since they carry proprioceptive information up to the brain; in some cases, the cause may instead be dysfunction of the various brain parts that receive that information, including the cerebellum, thalamus, and parietal lobes. Sensory ataxia presents
with an unsteady "stomping" gait with heavy heel strikes, as well as postural instability that is characteristically worsened when the lack of proprioceptive input cannot be compensated by visual input, such as in poorly lit environments. Doctors can evidence this during physical examination by having the patient stand with his / her feet together and eyes shut, which will cause the patient's instability to markedly worsen, producing wide oscillations and possibly a fall (this is called a positive Romberg's test). Worsening of the finger-pointing test with the eyes closed is another feature of sensory ataxia. Also, when the patient is standing with arms and hands extended toward the examiner, if the eyes are closed, the patient's finger will tend to "fall down" and be restored to the horizontal extended position by sudden extensor contractions ("ataxic hand").

The term vestibular ataxia is employed to indicate ataxia due to dysfunction of the vestibular system, which in acute and unilateral cases is associated with prominent vertigo, nausea and vomiting. In slow-onset, chronic bilateral cases of vestibular dysfunction, these characteristic manifestations may be absent, and dysequilibrium may be the sole presentation.

Exogenous substances that cause ataxia mainly do so because they have a depressant effect on central nervous system function. The most common example is ethanol, which is capable of causing overlapping cerebellar and vestibular ataxia. Other examples include both prescription drugs (e.g. most antiepileptic drugs have cerebellar ataxia as a possible unwanted effect) and recreational drugs (e.g. ketamine, PCP).

peripheral neuropathies may cause generalised or localised sensory ataxia (e.g. a limb only) depending on the extent of the neuropathic involvement. Spinal disorders of various types may cause sensory ataxia from the lesioned level below, when they involve the dorsal columns.

There is no specific treatment for ataxia as such, although there may be for the underlying cause. The disability of ataxia may be reduced by physical therapy, including exercises, along with leg braces or shoe splints, if foot alignment has been affected; a cane or walker is often used in the effort to prevent falls.